Crossbows are used for hunting nowadays. With the new fiberglass bows which are harder to bend, more pressure is exerted on the arrow by way of the string which translates into an arrow going over a much longer distance.
The problem with that is if an arrow is not present and that for some reason the string is triggered, that is called a dry-fire, and the bow, without the benefit of being slowed down by the departing arrow, unbends too rapidly, which can, more often than not, result in one or both sides of the bow to crack or even break.
In order to solve this problem, there are some crossbows that have an anti dry-fire system as part of their construction.
For example, one patent has the crossbow include a trigger mechanism comprised of a pivotal string catch member positioned relative to the trigger mechanism to selectively extend into a string catching position which will catch and retain the bowstring should it be released from the trigger mechanism without an arrow in a firing position in the crossbow. The string catch member includes an arrow contacting surface to engage an arrow positioned to be fired from the crossbow, wherein, movement of the arrow to the firing position causes the string catch member to be moved out of the string catching position. This allows selective release of the bowstring from the guide slot and prevents inadvertent release when no arrow is present in the firing position. The string catch member may be manually removed from the guide slot so as not to interfere with the bowstring, allowing a convenient mechanism to uncock the crossbow without firing.
But there does not appear to be any kits that allows for the retrofitting of an anti dry-fire device on existing crossbows.